Laser diodes are used extensively particularly in the fields of optical telecommunication and optical storage of information. Further, various applications of laser diodes are studied for example in the field of optical information processing such as optical computers.
In laser diodes, it is well known that the operational characteristics, particularly the threshold of laser oscillation, change with environmental temperature. Thus, in order to compensate for such a temperature-induced variation of the operational characteristics, the laser diode used for long range telecommunication purposes, such as the devices used for optical submarine cables, use a temperature regulation system such that the laser diode is held at a constant temperature.
On the other hand, there exist other applications of laser diodes wherein use of such a temperature regulation system is not possible or preferable. For example, the laser diodes used in the optical wiring of supercomputers have to be accommodated in a very limited space and the room for the temperature regulator is generally not available. Similarly, temperature regulation is not practical for the laser diodes that are used for the local area network (LAN), laser printers, and the like, because of the increased cost.
In order to avoid the unwanted temperature-induced fluctuation in the operation of laser diodes, conventional semiconductor optical sources use a feedback control of the optical beam, called automatic power control (APC), wherein the bias current is controlled such that the power of the output optical pulse of the laser diode is held constant. On the other hand, such an APC control has a problem in that, associated with the temperature-induced variation of the threshold of laser oscillation, the extinction ratio of the laser diode deteriorates with increasing temperature.
FIG. 1 shows the conventional APC control applied to laser diodes, wherein the horizontal axis represents the drive current while the vertical axis represents the corresponding output optical power. Further, the line designated as P.sub.L represents the output characteristics of the laser diode at a low temperature T.sub.L, while the line designated as P.sub.H represents the output characteristics of the same laser diode at a high temperature T.sub.H. It will be noted that the characteristic line P.sub.L indicates a threshold level I.sub.TH (T.sub.L) at the temperature T.sub.L, while the characteristic line P.sub.H indicates a threshold level I.sub.TH (T.sub.H) at the temperature T.sub.H.
Referring to FIG. 1, the drive current is supplied to the laser diode in the form of a current pulse I.sub.d, wherein the current pulse I.sub.d is biased at a level I.sub.bL that is set equal to the threshold level I.sub.TH (T.sub.L) at the temperature T.sub.L, such that the laser oscillation occurs with a minimum threshold power level when no current pulse I.sub.d is supplied. Further, the magnitude of the current pulse I.sub.d is set such that a predetermined output power is achieved in response to the drive current pulse I.sub.d at the foregoing low temperature. In the illustrated example, the magnitude of the current pulse I.sub.d is set to 5 mA.
In the APC control, the biasing I.sub.b of the current pulse I.sub.d is subjected to a feedback control that is achieved in response to the output power of the laser diode, such that a predetermined output power such as 0.5 mW is maintained even when the temperature of the laser diode changes. More specifically, the bias current I.sub.b added to the current pulse I.sub.d is changed such that the laser diode produces the foregoing predetermined output power in response to the current pulse I.sub.d,
When the biasing of the drive current pulse I.sub.d is set as such, there occurs a problem, when the temperature rises, in that the laser diode produces the optical output with a substantial power even when the current pulse I.sub.d is not supplied. In the illustrated example, the current pulse I.sub.d is biased at the level I.sub.bH of 20 mA at the temperature T.sub.H, and the laser diode produces the output optical power of about 0.25 mW in the absence of the input current pulse I.sub.d, Thereby, the extinction ratio of the output optical beam is inevitably deteriorated. Associated with the degradation of the extinction ratio, the S/N ratio of the optical information transmission achieved by the laser diode is deteriorated.
In order to maintain the output power of the optical beam constant, irrespective of a variation of the temperature, it is also possible to change the magnitude of the drive current pulse I.sub.d in response to the variation of temperature. For example, one may fix the level of the bias current at I.sub.TH (T.sub.L) and increase the magnitude of the current pulse I.sub.d with temperature such that the output power of 0.5 mW is secured even when the temperature increases. According to this approach, one can maintain a large extinction ratio. However, the foregoing approach has a drawback in that there tends to occur a delay in the timing of the optical pulse in correspondence to the interval necessary for the drive current to increase from the level I.sub.TH (T.sub.L) to the desired level such as 25 mA. In the illustrated example, it should be noted that the laser diode does not cause oscillation until the level of the drive current pulse I.sub.d reaches the level of 15 mA, and a time lag occurs in the optical pulse produced as a result of the laser oscillation.
In the applications where the use of expensive APC or automatic feedback control of the bias current is not desirable, therefore, one has to optimize the bias condition as well as the magnitude of the drive current pulse such that the problem of the degradation of the extinction ratio, as well as the problem of the oscillation delay of laser diode, is minimized.
FIGS. 3(A) and 3(B) show the eye patterns that are obtained as a result of the biasing shown in FIG. 2, wherein, in FIG. 2, it will be noted that the bias current I.sub.b is set between the level I.sub.TH (T.sub.L) and the level I.sub.TH (T.sub.H), and the magnitude of the drive current pulse I.sub.d is set such that the overall drive current I, defined as a sum of the bias current I.sub.b and the drive current pulse I.sub.d (I=I.sub.b +I.sub.d), exceeds the threshold I.sub.th (T.sub.H).
Referring to FIG. 3(A) showing the eye pattern at the temperature T.sub.L, the vertical axis represents the optical output power, while the horizontal axis represents the time. There, it will be noted that the laser diode produces the output optical power P.sub.0 in correspondence to the binary signal "0" wherein the drive current is set equal to the bias current I.sub.b. Further, in correspondence to the binary signal "1" wherein the drive current is set equal to the sum of the bias current I.sub.b and the drive current pulse I.sub.d, it will be noted that the laser diode produces the output optical power at the level P.sub.1.
With continuous operation of the laser diode, the temperature of the laser diode increases gradually from T.sub.L and reaches T.sub.H. Thus, the eye pattern shown in FIG. 3(A) shifts gradually to the right with time as shown in FIG. 3(B), and there appears a delay t.sub.d in the timing of the optical pulse as indicated in FIG. 3(B). When such a delay t.sub.d occurs, the time slot represented by the open area in the eye pattern inevitably decreases. It will be noted that the foregoing problem becomes particularly acute in the optical systems wherein a laser diode array, including a large number of laser diodes, is accommodated in a small space without temperature regulation as in the case of the foregoing optical wiring, optical local area network, optical print head of laser printers, and the like. In the optical systems of the foregoing type, a mere increase of the drive current pulse I.sub.d for increasing the extinction ratio may invite unwanted temperature increase that in turn results in an unwanted deterioration of the eye pattern characteristics.